The present invention is related to a sputtering process which is a technique whereby the small quantity of gas remaining in an evacuated (vacuum) chamber is placed in an electrical glow discharge state by electrical current (using the gas as a conductive path) passing from one electrode to another electrode within the vacuum chamber. In a preferred embodiment of the sputtering art, the voltage applied to the electrodes is a direct current (d.c.) voltage and therefore one of the electrodes becomes the cathode and the other electrode becomes the anode. The electrical glow discharge produces a large number of electrons and a large number of ions. The electrons of course, in accordance with the laws of physics, travel, or attempt to travel, toward the anode, while the ions travel, or attempt to travel, toward the cathode. A relatively high voltage difference is required across the electrodes to effect the gaseous glow discharge phenomenon. Accordingly the ions impinging upon the cathode do so with considerable momentum and dislodge atoms (particles) of cathode material from the cathode. The atoms or particles of the cathode material are ejected into the plasma and migrate throughout the vacuum chamber. These moving particles coat, or are deposited on, whatever surfaces they strike and such surfaces include the substrate and the anode.
While we are principally considering a d.c. sputtering technique, it should be noted that sputtering has been effected with an alternating current (a.c.) applied voltage at radio frequencies (RF). An a.c. voltage technique has some advantages such as the ability to sputter or deposit dielectric materials. However the a.c. technique has disadvantages as well, such as the necessity to provide impedance matching.
As the sputtering art developed it was discovered that the voltage difference required to start the gaseous glow discharge could be reduced and the number of ions for a given applied power could be increased (thereby reducing the coating time) by employing a magnetic field oriented to lie between 45.degree. to 90.degree. with respect to the electric field developed between the cathode and the anode. Further developments included using a planar plate, of the material to be deposited, (therefore as the cathode) in combination with magnetic pole pieces secured thereto. The magnetic field provided by the magnetic pole pieces effect an electron trap at and near the surface of the cathode which is being bombarded by the ions from the gaseous glow discharge. In this configuration the anode is located around the perimeter of the planar cathode and this arrangement will be referred to throughout this description as the perimeter anode arrangement. The electron trap technique greatly improves the rate of ionization and accordingly the rate of sputtering.
In all of the foregoing techniques very little consideration was given to the electron bombardment of the substrate to be coated. The electron impingement of a substrate can be undesirable, particularly if the substrate can be damaged by heat and/or radiation. It is true that using the magnetron mitigates the damage done by electron impingement because the electrons are initially confined by the magnetic field and the ratio of deposited material to electrons striking the substrate is higher than without the use of the magnetron. Nonetheless when the electrons pass outside of the magnetic field or electron trap (created by the flux of the magnets employed) there has been no effort to keep such electrons from striking the substrate. This problem is particularly acute when the substrates are dielectric specimens which are to be examined with scanning electron microscopes. The very fine detail of such a specimen is frequently lost if too much heat is generated or too much radiation is in effect as the result of electron impingement. The present invention provides a means to reduce the electron bombardment of the substrate while permitting a very high percentage of the dislodged cathode material to be deposited on the substrate.